Electric-power plants fired by coal or oil emit sulfur oxides, nitrogen oxides, and particulates. In industrialized countries, such plants account for up to 75% of the total of sulfur oxides, and, since the electric-power industry is rapidly proliferating, the potential increase of sulfur-oxide emissions is tremendous.
A number of measures have been adopted in an effort to control sulfur-oxide pollution. However, a number of technical problems stand in the way. In many existing power plants, low-sulfur coal cannot be burned without operational difficulties or without incurring high capital costs for furnace modifications. Sulfur can be removed from coal before burning, but the procedure is costly. The content can be cut in half by pulverizing the coal to the consistency of talcum powder and removing the pyrites (sulfur compounds) or by one-third by washing the coal and removing noncarbonaceous material. However, even with as much as 70% of the sulfur removed, the final coal product might still be classified as a high-sulfur fuel.
Several methods of removing sulfur from stack gases have been considered and utilized. In one technique, pulverized limestone or dolomite is added to the boiler charge, creating oxides that react with the sulfur oxides to form solid sulfite and sulfate particles that can be removed by electrostatic precipitation. In another process, catalytic conversion, the sulfur dioxide is converted to sulfur trioxide, which combines with water in the stack gas to form a sulfuric acid mist that can be trapped and eliminated. Another method is to produce sulfuric acid, which can be readily removed from the stack gas by the addition of an activated char, a carbonaceous material.
In most uses, the sulfur content of coal is objectionable in varying degrees. Part of the sulfur is associated with ash, and coal washing removes some sulfur along with the ash. Much sulfur, however, is more intimately associated with the coal substance itself and cannot be removed by washing. Since carbonization removes some sulfur, coke usually contains a lower percentage of sulfur than the coal from which it is made. During total gasification, most of the sulfur is converted into hydrogen sulfide, the form in which it can be readily separated from the gas. Extraction of coal with solvents produces an extract of relatively low sulfur content. Despite the use of these methods and considerable effort, no effective method has been devised to reduce the sulfur content substantially, particularly the portion closely associated with the coal substance.
It is an object of the present invention to reduce sulfur emissions from the combustion of high-sulfur coal.
It is a further object of the present invention to provide a process that reduces the ash from the combusted coal.
It is a further object of the present invention to provide a process that lowers the pH of the ash of the combusted coal.
It is still a further object of the present invention to provide a process for reducing sulfur emissions in an economic, efficient and easy-to-use operation.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification.